Sensory Neuropeptides Dictate Sex-Specific Synovial Immunity and Cartilage Degeneration in Aging Mice

Sensory neuropeptides, particularly Substance P (SP) and -calcitonin gene-related peptide (CGRP), are implicated in osteoarthritis (OA) pathogenesis. This study elucidates their specific roles in spontaneous, age-related OA. Male and female mice deficient in SP (Tac1-/-), CGRP (CGRP-/-), or both (DKO) were evaluated at 6, 12, and 18 months of age. Assessments included histological OARSI scoring for articular cartilage matrix structure, Luminex arrays for systemic serum cytokines, and flow cytometry for local synovial immune cell profiling. Wild type (WT) mice developed early-stage, age-related cartilage degradation, predominantly in the lateral compartment. Conversely, all neuropeptide-deficient strains exhibited significant structural protection against this process. Systemically, SP deficiency distinctly altered cytokine profiles (e.g., decreased IL-23, increased IP-10), whereas CGRP deficiency caused minimal systemic shifts, highlighting a disconnect between circulating markers and local joint preservation. Locally, flow cytometry revealed profound, sexually dimorphic, and age-dependent neuroimmune alterations. In young males, neuropeptide deficiency significantly reduced synovial macrophage counts to levels comparable to those of aged WT mice. Furthermore, male CGRP-/- mice exhibited an age-related accumulation of CD8+ cytotoxic T cells. In contrast to males, young WT females demonstrated higher baseline CD8+ T cell counts that declined with age, whereas these subpopulations remained persistently low in KO mice. SP and CGRP act as critical modulators of age-related cartilage degradation. Their absence provides robust structural protection mediated through highly localized, sexually dimorphic neuroimmune pathways. These findings emphasize the necessity of targeting the local joint microenvironment for future personalized, sex-specific OA therapies.